Mercury mass balances: A case study of two North Dakota power plants

The Energy & Environmental Research Center (EERC) conducted a mercury-sampl ing program to provide data on the quantity and forms of Hg emitted and on the Hg removal efficiency of the existing air pollution control devices at two North Dakota power plants-Milton R. Young Station and Coal Creek Statio n. Minnkota Power Cooperative, Great River Energy, the North Dakota Industr ial Commission, and EPRI funded the project. The primary objective was to o btain accurate measurements of Hg released from each plant, as verified by a material balance. A secondary objective was to evaluate the ability of a mercury continuous emission monitor (CEM) to measure total Hg at the stack. At both plants, speciated Hg measurements were made at the inlets and outle ts of both the electrostatic precipitators (ESPs) and the flue gas desulfur ization (FGD) systems. A Semtech Hg 2000 (Semtech Metallurgy AB) mercury CE M was used to measure the total Hg emissions at the stack in real time. Usi ng these measurements and plant data, the measured Hg concentrations in the coal, FGD slurries, and ESP ash, a Hg mass flow rate was calculated at eac h sampling location. Excellent Hg mass balances were obtained (+/-15%). It was also found that the Hg was mostly in the elemental phase (similar to 90 %), and the small amount of oxidized Hg that was generated was removed by t he FGD systems. Insignificant amounts of particulate-bound Hg were measured at both plants. However, 10-20% of the elemental Kg measured prior to the ESP was converte d to oxidized Hg across the ESP. The data show that, at these facilities, a lmost all of the Hg generated is being emitted into the atmosphere as eleme ntal Hg. Local or regional deposition of the Hg emitted from these plants i s not a concern. However, the Hg does become part of the global Hg burden i n the atmosphere. Also, the evidence appears to indicate that elemental Hg is more difficult to remove from flue gas than oxidized Hg is.